The present invention relates to a mass spectrometer and a method of mass spectrometry.
U.S. Pat. No. 6,373,052 (Micromass) discloses a method of correcting mass errors in mass spectra recorded by mass spectrometers that record single ion arrival events. The errors arise from a second ion arriving immediately after a first ion such that the electronic data handling and recording system is unable to record the second ion arrival event. The time period during which the electronic data handling and recording system is unable to record a second ion arrival event following a first ion arrival event is known as the deadtime.
The method disclosed in U.S. Pat. No. 6,373,052 comprises measuring the total number of ion arrival events which have been recorded within a known number of spectra for a mass spectral peak at a particular time of flight. An area and centroid correction are then applied to the observed mass spectral peak. The area and centroid correction are obtained from a predetermined correction table. The predetermined correction table is constructed using a plurality of computer simulations which predict the effect of the estimated detector deadtime on simulated mass peaks having peak shape functions approximating the mass spectral peaks to be corrected.
The use of a predetermined correction table enables corrections to be made very rapidly and avoids the need to store large amounts of raw mass spectral data.
The method disclosed in U.S. Pat. No. 6,373,052 however, makes no attempt to correct for distortions in centroid or area due to extending deadtime effects.
An ion arriving at an ion detector will cause the ion detector to suffer from a deadtime period wherein the subsequent arrival of ions during the deadtime period can not be recorded. If ions arrive during the deadtime period but do not extend the overall deadtime period any further then the deadtime is referred to as non-extending deadtime. However, if ions arrive during the deadtime period and cause the overall deadtime period to be extended further then the deadtime is referred to as extending deadtime.
Extending deadtime effects can result in inaccuracies in the reported centroid and area if individual peaks are separated by an amount approaching or less than the deadtime of the ion detector.
In addition, mass spectral peaks first need to be detected and identified before any form of correction procedure can be applied to the mass spectral data. The raw mass spectral data remains distorted and additional information which may be present in the raw mass spectral data such as peak shape information and mass resolution may also be distorted.
It is therefore apparent that peaks in raw distorted mass spectral data need to be detected. The shape and the width of peaks in the raw data will be dependent upon the intensity of the data if distortion due to the deadtime of the ion detector occurs. This may lead to errors in the consistency and accuracy of peak detection which in turn can compromise the consistency and accuracy of any correction applied.
A known method of correcting mass errors in mass spectral data obtained by a Time of Flight mass analyser is disclosed in ORTEC Application note AN57 and Chapter 8 of the ORTEC Modular Pulse-Processing Electronics catalogue. The disclosed method attempts to correct non-extending and extending deadtime effects using multi-channel scalars and time digitisers. These methods of correction are applied to the raw digitised data. The disclosed method does not consider however, that within one time digitisation period corresponding to the shortest time interval over which data may be recorded by the time digitiser used, more than one ion arrival event may occur in an individual time of flight spectrum. Consequently, insufficient intensity correction is applied to the data using the known method. This limits the ability of the known method to correct for deadtime distortions as the event arrival rate increases.
It is therefore desired to provide an improved method of distortion correction.